Engineering Materials and Failure Behavior

Introduction and context

Mechatronic products combine mechanical structures, actuators, sensors, electronics, software, and energy systems. Their reliability therefore depends strongly on materials. A material is not selected simply because it is “strong”; it must provide an appropriate combination of stiffness, strength, toughness, wear resistance, thermal stability, electrical behavior, manufacturability, cost, and environmental performance.

Material behavior arises from a chain of relationships:

composition and atomic structure → microstructure → processing → properties → component performance → failure mode

For example, changing carbon content and heat treatment in steel changes its phases and grain structure, which changes hardness, strength, ductility, fatigue resistance, and machinability. Similarly, polymer orientation during injection molding can produce directional stiffness and residual stress. Ceramic processing controls porosity, and porosity often determines fracture strength. Composite properties depend on fiber direction, interface quality, and manufacturing defects.

This chapter develops a structure–processing–property framework for metals, polymers, ceramics, composites, and smart materials. It then introduces material-selection indices and the principal failure mechanisms encountered in mechatronic systems: yielding, fracture, fatigue, creep, corrosion, wear, and environmentally assisted degradation.

Learning objectives

By the end of this chapter, you should be able to:

1. Relate atomic bonding, crystal structure, microstructure, and processing to engineering properties.
2. Compare the capabilities and limitations of metals, polymers, ceramics, composites, and smart materials.
3. Use basic performance indices to screen materials for stiffness-, strength-, and mass-sensitive designs.
4. Explain yielding, fracture, fatigue, creep, corrosion, and wear using appropriate models.
5. Identify interactions among material, geometry, loading, environment, and manufacturing defects.
6. Select materials for mechatronic components using technical, economic, environmental, and ethical criteria.

1. From structure to engineering properties

1.1 Atomic bonding and classes of material

Metallic bonding involves positively charged atomic cores surrounded by mobile electrons. It explains electrical and thermal conductivity, ductility, and the ability of metals to undergo plastic deformation. Covalent bonding involves directional sharing of electrons and commonly produces high stiffness, hardness, and thermal stability. Ionic bonding produces strong electrostatic attraction; many ionic solids are hard but brittle. Polymers also contain covalent bonds along molecular chains, but weaker secondary bonds between chains permit viscoelastic deformation.

The major material families have characteristic trade-offs:

1.2 Crystalline structure and defects

In crystalline solids, atoms occupy a repeating lattice. Common metallic structures include body-centred cubic (BCC), face-centred cubic (FCC), and hexagonal close-packed (HCP). FCC metals generally have many active slip systems and are therefore ductile. HCP metals have fewer easy slip systems at room temperature and may be less ductile unless texture or temperature assists deformation.

Real materials contain defects. Point defects include vacancies and substitutional or interstitial atoms. Dislocations are line defects that enable plastic slip at stresses far below those required to shear an entire perfect crystal. Grain boundaries interrupt slip and can strengthen a metal, but they may also act as paths for corrosion or diffusion. Voids, inclusions, pores, and cracks reduce fatigue and fracture resistance.

For many metals, grain refinement increases yield strength. A commonly used empirical relationship is the Hall–Petch relation:

Here σ_y is yield stress in Pa, σ₀ is a friction stress in Pa, k_y is a material constant with units Pa·m^{1/2}, and d is average grain diameter in m. The relation is an empirical approximation for polycrystalline materials over a range of grain sizes; it is not universally valid at extremely fine or very coarse scales.

1.3 Processing and microstructure

Processing determines phase constitution, grain size, texture, residual stress, porosity, surface condition, and internal defects. Important processes include:

• Casting: economical for complex shapes, but may produce shrinkage cavities, segregation, and coarse grains.
• Wrought deformation: rolling, forging, and extrusion refine structure and can create directional properties.
• Heat treatment: annealing reduces residual stress; quenching can form hard martensite in suitable steels; tempering improves toughness; precipitation hardening forms fine strengthening particles.
• Powder processing and additive manufacturing: enable complex geometry but require control of porosity, thermal gradients, residual stress, and surface roughness.
• Polymer molding: controls molecular orientation, crystallinity, weld lines, and residual stress.
• Ceramic sintering: reduces porosity through diffusion but may cause shrinkage and distortion.
• Composite lay-up or winding: establishes fiber orientation and therefore the load-carrying direction.

A design specification should therefore identify not only a material grade but also its processing route, heat treatment, surface condition, joining method, and inspection requirements.

2. Material families in mechatronic engineering

2.1 Metals and alloys

Pure metals are often too soft or weak for engineering use, so alloying and processing are essential. Steels offer a wide range of strength, toughness, weldability, and cost. Aluminum alloys provide low density and good corrosion resistance, although their elastic modulus is only about one-third that of steel. Copper alloys are valuable for electrical conductivity, thermal management, and wear-resistant components. Nickel- and titanium-based alloys retain strength at elevated temperature but are more expensive and difficult to process.

Metals are usually isotropic at the macroscopic scale when processed conventionally, but rolling, forging, welding, and additive manufacturing can introduce anisotropy. Surface treatments such as carburizing, nitriding, anodizing, plating, and shot peening modify wear, corrosion, and fatigue performance.

2.2 Polymers

Thermoplastics soften when heated and can be remelted; thermosets form permanent cross-linked networks; elastomers have lightly cross-linked chains and recover large strains. Polymer behavior is strongly dependent on temperature, time, strain rate, molecular weight, crystallinity, and moisture.

The simplest linear elastic relation is valid only over a limited range:

where σ is normal stress in Pa, E is Young’s modulus in Pa, and ε is dimensionless normal strain. For polymers, E is often time- and temperature-dependent, so the equation should not be used blindly for long-duration loading.

Viscoelasticity combines elastic energy storage with viscous dissipation. Under constant stress, strain may increase with time (creep); under constant strain, stress may decrease (relaxation). A useful engineering model is the Maxwell model, consisting of a spring and dashpot in series. For a constant stress σ₀, its idealized creep response is:

Here E is spring modulus in Pa, η is dashpot viscosity in Pa·s, t is time in s, and the model assumes small strain, linear behavior, and constant temperature. Real polymers generally require more sophisticated generalized models.

2.3 Ceramics and glasses

Ceramics have strong ionic or covalent bonds and are often stiff, hard, wear resistant, electrically insulating, and stable at high temperature. Their critical weakness is brittle fracture. A ceramic component can have high nominal strength yet fail suddenly because a small surface flaw amplifies local stress.

Ceramic strength is statistically variable because flaw populations vary. Grinding, machining, thermal shock, impact, and tensile residual stress can be more important than nominal material strength. Design should minimize tensile stress, protect surfaces, use compressive surface layers where appropriate, and specify proof testing or non-destructive inspection when justified.

2.4 Fiber-reinforced composites

A composite consists of at least two distinct phases whose combined performance is useful: typically strong fibers in a polymer, metal, or ceramic matrix. Fibers carry much of the axial load, while the matrix transfers load between fibers, protects them, and resists transverse or shear loading.

For a continuous, aligned, long-fiber composite loaded parallel to the fibers, the approximate rule of mixtures is:

where E_L is longitudinal modulus, E_f and E_m are fiber and matrix moduli, and V_f and V_m are their volume fractions, with V_f + V_m = 1. This is an upper-bound approximation requiring good bonding, aligned continuous fibers, and elastic loading in the longitudinal direction. It does not predict transverse modulus or laminate damage.

Failure can occur by fiber breakage, matrix cracking, interface debonding, delamination, or local buckling. Consequently, composite design requires attention to joints, holes, impact damage, moisture, ultraviolet radiation, and inspection access.

2.5 Smart materials

Smart materials respond to an external stimulus and can sense, actuate, or adapt. Examples include:

• Piezoelectric ceramics: generate charge under mechanical stress and deform under electric field; useful for precision positioning and vibration sensing.
• Shape-memory alloys: recover a trained shape through martensitic phase transformation; useful for compact actuators but limited by hysteresis, heat transfer, and cycling.
• Magnetostrictive materials: change dimensions in a magnetic field and can provide robust actuation.
• Electroactive polymers: produce large strains at relatively low mass but may require high electric fields and careful insulation.
• Phase-change and thermochromic materials: respond to temperature and can support thermal management or indication.

Smart-material performance is system-level: the actuator, sensor, power electronics, controller, thermal path, and mechanical load must be designed together. A material with a high free strain may deliver little useful stroke under load, while a material with excellent sensitivity may be unsuitable in a harsh environment.

3. Material selection using performance indices

Material selection begins with a function–objective–constraint description. For example: “support a prescribed bending load, minimize mass, maintain a deflection below a limit, operate from −20 °C to 80 °C, resist corrosion, and be recyclable.” A database ranking by strength alone cannot solve this problem.

3.1 Basic mechanical quantities

Normal stress and strain for a prismatic member under axial load are:

F is axial force in N, A is cross-sectional area in m², ∆L is extension in m, and L₀ is original length in m. These definitions assume a sufficiently uniform member and nominal engineering stress and strain.

Strength is resistance to yielding or fracture. Stiffness is resistance to elastic deformation and is governed by modulus and geometry. Toughness is energy absorbed before fracture. Hardness is resistance to indentation or scratching. These are not interchangeable.

3.2 Specific properties and indices

When mass matters, specific stiffness and specific strength are useful:

where ρ is density in kg·m⁻³. For a simply supported beam of fixed length and width carrying a central load, minimizing mass while satisfying a deflection constraint leads to a stiffness-related index proportional to E^1/3/ρ. The exact exponent depends on geometry, loading, and the chosen constraint; indices must not be transferred between different structural problems without derivation.

Worked example: screening a beam material

An equal-volume comparison is made between aluminum with E = 70 GPa, ρ = 2700 kg·m^-3 and steel with E = 200 GPa, ρ = 7800 kg·m^-3. Their specific stiffnesses are:

Aluminum: 70×10⁹/2700 ≈ 25.9×10⁶ m²/s².

Steel: 200×10⁹/7800 ≈ 25.6×10⁶ m²/s².

For equal volume, steel is much stiffer because its modulus is higher, but aluminum is much lighter. For a mass-constrained beam, the near-equality of specific stiffness suggests that both may be competitive; geometry, joining, corrosion, cost, and manufacturing then become decisive. This example illustrates why density alone or modulus alone is inadequate.

4. Failure behavior and assessment

4.1 Yielding and plastic collapse

Yielding begins when stress reaches a condition for significant permanent deformation. For ductile isotropic metals under multiaxial loading, the von Mises equivalent stress is:

Yielding is predicted when σ_v ≥ σ_y, where σ₁, σ₂, and σ₃ are principal stresses and σ_y is uniaxial yield stress. This criterion is mainly appropriate for ductile metals under monotonic or appropriately interpreted loading. It is not a fracture criterion for brittle ceramics and does not by itself describe fatigue or creep.

A design factor against yielding may be written:

where σ_v,max is the maximum calculated equivalent stress. The selected factor must account for uncertainty, consequence of failure, model error, manufacturing variation, and applicable standards.

4.2 Fracture and stress concentration

Linear elastic fracture mechanics characterizes a crack using the stress-intensity factor:

K_I is mode-I stress intensity in Pa·m^{1/2}, Y is a dimensionless geometry factor, σ is nominal tensile stress in Pa, and a is crack size in m. Fracture is expected when K_I ≥ K_IC, where K_IC is plane-strain fracture toughness. The model assumes predominantly linear elastic behavior, a sufficiently sharp crack, appropriate specimen constraint, and valid geometry calibration.

Notches increase local stress. A theoretical elastic stress concentration factor is K_t = σ_local/σ_nominal. A rounded fillet generally reduces K_t, while a sharp corner, keyway, hole, thread root, or machining mark increases it. A ductile material may blunt a notch through local yielding; a brittle material may remain highly sensitive to it.

4.3 Fatigue

Fatigue is progressive damage under repeated or fluctuating loading, often at stresses below the static yield strength. Important variables include stress amplitude, mean stress, surface finish, residual stress, size, temperature, environment, frequency, and defects.

For fully reversed loading, a stress-life relation often used for high-cycle fatigue is Basquin’s form:

where σ_a is stress amplitude, σ'_f is a fitted fatigue-strength coefficient, N_f is cycles to failure, and b is a fitted negative exponent. It is empirical and must be calibrated for the material, surface, mean stress, and test conditions. Mean stress corrections, such as Goodman-type approaches, require careful application and do not replace component testing.

Fatigue design practice includes reducing stress concentrations, improving surface finish, controlling tensile residual stress, avoiding fretting, selecting appropriate preload, and inspecting critical components. For rotating shafts and robotic joints, fatigue evaluation should include start–stop cycles, imbalance, backlash impacts, and bearing loads rather than only nominal steady torque.

4.4 Creep and stress relaxation

Creep is time-dependent deformation under sustained stress, especially important above roughly 0.4 of a material’s melting temperature in kelvin for many crystalline materials; the threshold varies widely. It is also important in polymers at much lower service temperatures relative to their glass-transition temperature.

A typical creep curve has primary creep with decreasing rate, secondary creep with approximately constant rate, and tertiary creep with accelerating damage. A simplified steady-state relation is Norton’s law:

ε̇_s is steady creep strain rate, A and n are material constants, σ is stress, Q is activation energy in J·mol⁻¹, R is the gas constant 8.314 J·mol⁻¹·K⁻¹, and T is absolute temperature in K. The relation is empirical over a specified stress–temperature range; extrapolation outside that range can be unsafe.

4.5 Corrosion and environmental degradation

Corrosion is chemical or electrochemical deterioration. Uniform corrosion reduces section thickness, while pitting, crevice corrosion, galvanic corrosion, intergranular corrosion, stress-corrosion cracking, and corrosion fatigue can produce localized and sometimes sudden failure.

Galvanic corrosion requires electrical contact between dissimilar metals and an electrolyte. Risk is increased when a small anodic area is coupled to a large cathodic area. Practical controls include compatible material pairs, electrical isolation, coatings, drainage, seal design, cathodic protection, and environmental control.

Polymers may suffer oxidation, hydrolysis, solvent attack, ultraviolet degradation, plasticization, or embrittlement. Composites may absorb moisture, lose matrix stiffness, and develop swelling or interface damage. Ceramics are generally corrosion resistant but can dissolve, react at high temperature, or suffer thermal shock. Environmental qualification must therefore reproduce the actual temperature, humidity, chemicals, radiation, electrical potential, and mechanical cycling.

4.6 Wear and tribological failure

Wear is material loss or surface damage caused by relative motion. Adhesive wear occurs when contacting asperities bond and tear; abrasive wear results from hard particles or asperities; fatigue wear arises from repeated subsurface stress; erosion involves fluid-borne particles or droplets.

A widely used screening relation for adhesive or abrasive wear is Archard’s law:

V is wear volume in m³, k is an empirical dimensionless wear coefficient, W is normal load in N, L is sliding distance in m, and H is hardness in Pa. It assumes a defined wear regime and is not reliable across changes in lubrication, temperature, materials, surface finish, or wear mechanism.

5. Integrated material selection procedure

A defensible selection process is:

1. Define function, loads, motion, interfaces, temperature, environment, life, safety requirements, and production volume.
2. Identify failure modes using stress analysis, fatigue assessment, fracture assessment, corrosion review, and hazard analysis.
3. Establish constraints such as maximum mass, stiffness, electrical insulation, biocompatibility, fire behavior, and regulatory requirements.
4. Screen material families using performance indices and eliminate incompatible candidates.
5. Select grades and processing routes, including heat treatment, surface treatment, joining, and tolerances.
6. Verify properties using reliable data for the actual condition, not generic handbook values alone.
7. Prototype and test under representative loads and environments.
8. Document traceability, inspection, maintenance, recycling, and end-of-life requirements.

The final decision should use a weighted trade-off rather than a single score. High-risk applications require independent design review and evidence that the selected material remains safe under credible defects and misuse.

6. Illustrative case: material selection for a collaborative robot wrist housing

Background

Consider an illustrative design study for a small collaborative robot. The wrist housing supports a gearbox, protects wiring and sensors, and is handled near people. The housing must be light, stiff enough to preserve positioning accuracy, corrosion resistant, electrically safe around embedded electronics, and manufacturable in moderate production volume.

Problem

A steel housing provides stiffness but increases moving mass and actuator demand. A polymer housing is light and electrically insulating but may creep around fasteners and soften near motors. A carbon-fiber composite is light and stiff along selected directions but complicates threaded inserts, impact inspection, and recycling.

Analysis

The dominant failure modes are excessive deflection, fastener pull-out, fatigue at mounting bosses, impact damage, thermal distortion, and environmental degradation. A modal analysis is required because low housing stiffness can shift resonances into the servo bandwidth. Local bearing seats may need metallic inserts even if the main shell is polymer or composite.

Approach and solution

An illustrative hybrid design uses a glass-fiber-reinforced engineering thermoplastic shell with aluminum inserts at bearing and fastener interfaces. Ribbing increases second moment of area without a proportional increase in mass. Brass or steel threaded inserts distribute load and reduce creep. The polymer is selected using long-term creep data at the maximum service temperature, not short-term tensile modulus alone. Sealing features prevent moisture ingress, and accelerated thermal-humidity testing is used to check dimensional stability.

The design team compares the hybrid polymer, aluminum, and composite options using mass, stiffness, cost, assembly time, recyclability, electromagnetic compatibility, and repairability. Finite-element results are verified with static, vibration, drop, thermal-cycle, and endurance tests. Fasteners are installed with controlled torque, and the design avoids sharp internal corners that would create stress concentrations.

Outcome

The hybrid solution reduces moving mass while retaining durable interfaces. Its success depends on inserts, rib geometry, creep-aware preload design, and environmental validation. The case demonstrates that material selection is inseparable from geometry, processing, joining, control performance, and maintenance.

Learning points

• The best bulk material may not be the best interface material.
• Long-term properties can govern a design that appears safe in a short static test.
• Lightweighting must consider vibration, impact, repair, and end-of-life behavior.
• Material data must correspond to temperature, moisture, orientation, processing, and aging conditions.

7. Sustainability, ethics, and contemporary practice

Material decisions affect extraction, energy use, emissions, toxicity, worker safety, and end-of-life recovery. A lightweight component may reduce operational energy but require energy-intensive processing. Composites can reduce mass yet be difficult to recycle. Aluminum recycling can be advantageous, but contamination and collection systems matter. Polymers may release hazardous substances during processing or combustion; coatings and plating may create occupational and environmental hazards.

Engineers should use life-cycle thinking: assess raw-material extraction, manufacturing, transportation, use-phase energy, maintenance, reuse, recycling, and disposal. Design for disassembly, repair, replaceable wear parts, standardized fasteners, material labeling, and reduced material diversity where feasible.

Digital tools and AI can support material databases, process optimization, defect detection, and predictive maintenance. They do not remove the need for validated data, physical testing, explainable assumptions, or engineering accountability. A machine-learning prediction trained on polished laboratory data may fail for a rough, damaged, temperature-cycled production part. Safety-critical decisions require traceable evidence and human review.

Best practice includes compliance with applicable material, pressure, machinery, electrical, fire, chemical, and occupational-safety standards; clear control of material certificates; configuration management; and documented change approval. Ethical practice requires conservative treatment of uncertainty when failure could injure users or workers.

Challenges and limitations

Material properties are not single universal numbers. They vary with direction, temperature, strain rate, humidity, processing, defects, specimen size, and test method. Handbook values may represent ideal laboratory specimens and may not apply to a finished component. Safety factors cannot compensate indefinitely for poor modeling, unknown defects, or an unsuitable failure mechanism. Numerical simulation is powerful but must be calibrated and verified. Finally, a design optimized for strength may be poor in fatigue, corrosion, maintainability, sustainability, or total cost.

Chapter Summary

Engineering performance follows a structure–processing–property chain. Atomic bonding, crystal structure, defects, phases, grains, molecular architecture, porosity, fiber orientation, and interfaces determine how materials respond to load and environment. Metals provide versatile strength and toughness; polymers offer low mass and manufacturability but are sensitive to time and temperature; ceramics provide hardness and stability but are flaw-sensitive; composites provide directional specific performance but can delaminate; smart materials couple mechanical behavior with electrical, thermal, magnetic, or chemical stimuli.

Material selection requires a function–objective–constraint statement, appropriate performance indices, and verification under realistic conditions. Yielding, fracture, fatigue, creep, corrosion, and wear are distinct failure mechanisms and require different models and tests. Geometry, surface condition, joining, processing defects, environment, and maintenance can dominate nominal material properties. Sustainable and ethical engineering extends selection beyond initial strength and cost to life-cycle impacts, safety, repairability, traceability, and responsible use of digital tools.

Key Terms

Atomic bonding: Interactions that hold atoms together and influence material behavior.

Microstructure: Internal arrangement of phases, grains, defects, and interfaces.

Dislocation: Line defect that enables plastic slip in crystalline materials.

Yield strength: Stress associated with the onset of specified permanent deformation.

Toughness: Energy absorbed before fracture.

Hardness: Resistance to indentation, scratching, or local plastic deformation.

Viscoelasticity: Time-dependent behavior combining elastic and viscous responses.

Creep: Time-dependent deformation under sustained load.

Stress relaxation: Decrease in stress under approximately constant strain.

Composite: Material formed from distinct phases that act together to provide useful properties.

Anisotropy: Dependence of properties on direction.

Smart material: Material that responds functionally to an external stimulus.

Stress concentration: Local amplification of stress caused by geometry or defects.

Fracture toughness: Resistance to crack extension, commonly represented by K_IC.

Fatigue: Progressive damage caused by repeated or fluctuating loading.

Corrosion: Chemical or electrochemical deterioration of a material.

Wear: Progressive surface damage or material loss caused by contact and motion.

Performance index: Combination of properties used to rank materials for a defined design objective.

Life-cycle thinking: Evaluation of impacts from material extraction through end of life.

Review Questions

5 Short Answer Questions

1. How does processing influence the microstructure and properties of a metal?
2. Distinguish stiffness, strength, hardness, and toughness.
3. Why are polymers generally more sensitive to time and temperature than metals?
4. Name two failure modes specific to or especially important in fiber-reinforced composites.
5. What environmental conditions can accelerate corrosion or polymer degradation?

5 Long Answer Questions

1. Explain the structure–processing–property relationship using one metal and one polymer example.
2. Compare metals, ceramics, polymers, and composites for a lightweight robotic arm.
3. Explain the assumptions and limitations of linear elastic fracture mechanics.
4. Discuss how creep, fatigue, and corrosion can interact in a mechatronic actuator.
5. Develop a systematic material-selection procedure for an automated manufacturing fixture.

3 Analytical / Application Questions

1. A shaft contains a keyway and is subjected to fluctuating torque. Identify likely failure mechanisms and propose geometric, material, surface, and inspection controls.
2. A polymer gear operates continuously near a warm motor. Explain what data and tests are needed before approving the design, including creep, wear, temperature, and lubrication considerations.
3. Select and justify a material family for a drone actuator bracket where low mass, vibration resistance, corrosion resistance, manufacturability, and end-of-life impact are all important. State the trade-offs and verification tests.

Multiple Choice Questions with Answers

Question 1

Which processing change most directly refines the grain structure of a conventional metal component?

A. Controlled plastic deformation followed by suitable recrystallization treatment

B. Increasing the surface roughness after machining

C. Replacing tensile loading with electrical insulation

D. Removing all alloying elements

Correct Answer: A. Controlled plastic deformation followed by suitable recrystallization treatment

Question 2

For a ductile isotropic metal, the von Mises criterion predicts yielding when:

A. The density reaches a critical value

B. The equivalent stress reaches the yield strength

C. The crack length becomes zero

D. The elastic modulus becomes temperature-independent

Correct Answer: B. The equivalent stress reaches the yield strength

Question 3

In the relation K_I = Yσ√(πa), which variable represents crack size?

A. Y

B. σ

C. a

D. K_I

Correct Answer: C. a

Question 4

Which material family is generally most sensitive to long-term creep at ordinary engineering temperatures?

A. Engineering polymers

B. Tool ceramics

C. Glass fibers alone

D. Hardened carbide

Correct Answer: A. Engineering polymers

Question 5

The longitudinal rule of mixtures for a continuous aligned composite assumes that:

A. Fibers are randomly oriented and disconnected

B. The material is loaded perpendicular to all fibers

C. Fiber and matrix have compatible deformation and effective load transfer

D. The matrix carries no shear or protective function

Correct Answer: C. Fiber and matrix have compatible deformation and effective load transfer

Question 6

A small anodic metal area electrically coupled to a large cathodic area generally creates:

A. Lower local corrosion risk at the anode

B. Higher corrosion current density at the anode

C. Zero electrochemical activity

D. Only mechanical fatigue with no corrosion

Correct Answer: B. Higher corrosion current density at the anode

Question 7

Which design change usually reduces an elastic stress concentration at a shaft shoulder?

A. Using a sharper corner

B. Adding a larger-radius fillet

C. Adding a deeper notch

D. Reducing the shaft diameter abruptly

Correct Answer: B. Adding a larger-radius fillet

Question 8

For aluminum with E=70 GPa and ρ=2700 kg·m^-3, the approximate specific stiffness is:

A. 2.59×10⁴ m²/s²

B. 2.59×10⁶ m²/s²

C. 25.9×10⁶ m²/s²

D. 259×10⁹ m²/s²

Correct Answer: C. 25.9×10⁶ m²/s²

Question 9

Which statement best describes fatigue?

A. Failure only above the static ultimate strength

B. Progressive damage under repeated or fluctuating loading

C. Instantaneous melting caused by friction

D. Permanent deformation only under constant temperature

Correct Answer: B. Progressive damage under repeated or fluctuating loading

Question 10

Why are material certificates and configuration control important in safety-critical products?

A. They eliminate the need for testing

B. They ensure traceability of material condition and approved changes

C. They guarantee that no component can corrode

D. They replace engineering judgment with a single property value

Correct Answer: B. They ensure traceability of material condition and approved changes